The present invention relates to a spark plug and a method of manufacturing the same, in which at least one of electrodes of center electrode and earth electrode, which are disposed so as to oppose to each other, is formed as a base material and a tip as a discharge material formed of noble metal or its alloy is welded to one surface of the base material and fixed thereto and also relates to a method of manufacturing such spark plug, which is particularly usable in a severe environment of such as thermal load of a cogeneration engine or like.
In a known art, life time and performance of a spark plug have been improved by welding and fixing a tip, formed of a noble metal or its alloy such as Ir, Pt or like as a discharge member performing spark discharge, to a center electrode or an earth electrode (base material). A resistance welding is generally utilized for such tip joining method in a viewpoint of easy manufacturing, low cost, or like. However, in the case of using Ir alloy as the tip, since the Ir alloy has a linear expansion coefficient with respect to the base material larger than the case of Pt alloy, it is difficult to ensure joining reliability between the tip and the base material in the resistance welding method.
Because of the reason mentioned above, in the prior art, in the case where the tip formed of the Ir alloy is joined (welded), an alloy layer (relaxation layer) formed by melting the Ir alloy and the base material (Ni alloy or like) between these materials is formed by means of laser welding thereby to attenuate a thermal stress applied to the joining portion between the tip and the base material, thus ensuring the joining reliability between the tip and the base material.
However, in an experience of the inventor of the subject application, the thermal stress has been increased in the case of using the spark plug under a condition of large-sized tip or severe thermal environment (for example, in a cogeneration engine, temperature of the center electrode increases to about 950xc2x0 C.) regardless of the kind of the welding methods such as laser welding, resistance welding or like. In an adverse case, the tip may fall down from the base material, thus providing a problem.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a spark plug capable of ensuring the joining (welding) reliability between a tip and a base material even in a severe thermal stress or load environment.
Another object of the present invention is to provide a method of manufacturing the spark plug of the character mentioned above in a manner easily performed.
These and other objects can be achieved according to the present invention by providing, in one aspect, a spark plug for an engine comprising:
a mounting member made of a conductive material which is mounted to an engine;
an insulator disposed inside the mounting member and formed with an axial hole;
a center electrode formed of a metal material and disposed in the axial hole of the insulator in a manner insulated from the mounting member; and
an earth electrode disposed so as to oppose to the center electrode,
at least one of the center electrode and earth electrode being formed as a base material having one surface to which a tip composed, as a discharge member, of a noble metal or alloy thereof is welded and fixed thereto through a fused portion,
wherein the fused portion between the tip and the base material has an area of a maximum sectional area portion of a size of not more than 1.5 times a sectional area of a portion of the tip positioned at a boundary portion to the fused portion, and the sectional area of the tip positioned at the boundary portion is not less than 2 mm2 and not more than 7 mm2.
Further, it is to be noted that the one surface of the base material to which the tip has been welded means or represents the surface of the base material in the state before the welding to the tip because, after the welding, this surface may not be clear for the formation of the fused portion, for example.
According to the invention of this aspect, the fused portion between the tip and the base material has an area of a maximum sectional area portion of a size of not more than 1.5 times a sectional area of a portion of the tip positioned at a boundary portion to the fused portion, and accordingly, the thermal stress to the fused portion can be reduced and even if the spark plug is used under the severe thermal load condition of a cogeneration engine, for example, the joining reliability between the tip and the base material can be effectively ensured. This effect can be further enhanced by the definition that the sectional area of the tip positioned at the boundary portion is not less than 2 mm2 and not more than 7 mm2.
In a preferred example of the above aspect, the fused portion has a minimum sectional area portion having an area of a size of not less than 0.6 time the sectional area of the tip positioned at the boundary portion to the fused portion. The base material has a sectional area, as a small sectional area portion, along one surface thereof, which has a size of not more than 1.5 times the sectional area of the portion of the tip positioned at the boundary portion to the fused portion, the small sectional area portion being formed so as to provide a formation length of not more than 2.0 mm in a direction normal to the one surface of the base material from the boundary portion between the base material and the fused portion.
The tip is fixed to the one surface of the base material through a laser welding. The tip is formed of an alloy including Ir of not less than 50 weight %.
According to the preferred example, the formation of the small sectional area portion makes reduce the size of the base material with respect to the fused portion, thus reducing the thermal stress. In the experiment of the inventors, the following facts have been revealed.
The small sectional are portion has a fine shape in comparison with other portions of the base material such as center electrode and/or earth electrode. If the formation length exceeds 2.0 mm, the base material will be made too fine to obtain a good thermal conductivity and the temperature increasing of the base material will increase the thermal stress to the fused portion as the result. Therefore, it is difficult to ensure the reliable joining performance. On the contrary, in the case of the formation length of not more than 2.0 mm, the temperature increasing of the base material will be suppressed and the thermal stress to the fused portion will be hence reduced, thus being advantageous.
Furthermore, according to the laser welding, the tip can be firmly fixed to the surface of the base material with a large thickness (i.e. for example, several hundreds xcexcm to 1 mm) of the fused portion in comparison with the resistance welding. thus the tip being surely fixed.
Still furthermore, by forming the tip by using a metal material of such as Ir or its alloy of the weight % mentioned above, the tip can achieve more effective function with respect to the base material having a large difference of linear expansion coefficient.
In another aspect of the present invention, there is also provided a spark plug for an engine comprising:
a mounting member made of a conductive material which is mounted to an engine;
an insulator disposed inside the mounting member and formed with an axial hole;
a center electrode formed of a metal material and disposed in the axial hole of the insulator in a manner insulated from the mounting member; and
an earth electrode disposed so as to oppose to the center electrode,
at least one of the center electrode and earth electrode being formed as a base material having one surface to which a tip composed, as a discharge member, of a noble metal or alloy thereof is welded and fixed thereto through a fused portion,
wherein the fused portion between the tip and the base material has a sectional area having shape and size substantially the same as those of a sectional area of a portion of the tip positioned at a boundary portion to the fused portion.
In the above aspect, if the tip and the fused portion have substantially the same shapes and/or dimensions in their sectional areas at the boundary portion between these tip and the fused portion, there will exist an extruded (extruding) portion of the tip or fused portion, and an edge portion will be formed at such extruded portion, to which the thermal stress is likely concentrated.
However, according to the present invention of the above aspect, since the tip and the fused portion have the same shape and/or dimension in their sectional area at the boundary portion therebetween, any edge portion of the type mentioned above never exist, thus reducing the thermal stress and ensuring the joining reliability between the tip and the base material even in the use under the severe thermal load condition such as in a coegeneration engine.
In a preferred example of the present invention of the above aspect, the base material has a sectional area, as a same-shaped portion, along one surface thereof, which has shape and size substantially the same as those of the sectional area of the fused portion, the same-shaped portion being formed so as to provide a formation length of not more than 2.0 mm in a direction normal to the one surface of the base material from the boundary portion between the base material and the fused portion.
The tip is fixed to the one surface of the base material through a laser welding. The tip is formed of an alloy including Ir of not less than 50 weight %.
According to such preferred example, the advantageous effects mentioned above in this and aforementioned aspects can be also achieved in more effective manner.
In a further aspect of the present invention, there is also provided a method of manufacturing a spark plug for an engine of the structures mentioned above in the one and another aspect of the present invention,
the method comprising the steps of;
welding the tip to the one surface of the base material; and
working the fused portion between the tip and the base material so as to provide a predetermined shape.
In a preferred example, the fused portion between the tip and the base material has an area of a maximum sectional area portion having a size of not more than 1.5 times a sectional area of a portion of the tip positioned at a boundary portion to the fused portion. The sectional area of the tip positioned at the boundary portion is not less than 2 mm2 and not more than 7 mm2.
The fused portion between the tip and the base material has a sectional area having shape and size substantially the same as those of a sectional area of a portion of the tip positioned at a boundary portion to the fused portion. The fused portion and the base material adjacent to the fused portion is subjected to a shaping working so as to provide a predetermined shape of the base material. The tip is fixed to the one surface of the base material through a laser welding.
According to the preferred example of this aspect, the shape and size of the sectional area of the fused portion between the tip and the base material can be adjusted and regulated, the desired fused portion can be formed, and hence, the spark plug of the desired structure can be manufactured.
The nature and further characteristic features of the present invention will be made further clear from the following descriptions in relation to effected experiments with reference to the accompanying drawings.